p53, also known as TP53
or tumor protein
(EC :2.7.1.37) is a gene
that codes
for a protein
that regulates the cell
cycle and hence functions as a tumor
suppression. It is very important for cells in multicellular
organisms to suppress cancer.
P53 has been described as "the guardian of the genome",
referring to its role in conserving stability by preventing genome
mutation (Strachan
and Read, 1999). The name is due to its molecular
mass: it is in the 53 kilodalton
fraction of cell proteins.

2.HISTORY

p53 was identified in 1979
by Arnold
Levine,David
Lane and William
Old,working at Princeton
University, Dundee
University (UK) and Sloan-Kettering
Memorial Hospital, respectively. It had been hypothesized to exist
before as
the target of the SV40
virus, a strain that induced development of tumors.Although it was
initially
presumed to be an oncogene,
its character as a tumor suppressor gene was revealed in 1989.In 1993,
p53 protein
has been voted molecule
of the year by the Science
magazine

3. GENE

The human p53 gene is located on the seventeenth chromosome
(17p13.1).

4. STRUCTURE

The p53 protein is a phosphoprotein made of 393 amino acids. It
consists of four
units (or domains):

A domain that
activates transcription factors.

A domain that
recognizes specific DNA sequences (core domain).

A domain that is
responsible for the tetramerization of the protein.

A domain that
recognized damaged DNA, such as misaligned base pairs or
single-stranded DNA.

Wild-type p53 is a labile protein, comprising folded and unstructured
regions
which function in a synergistic

manner (Bell et al. 2002).p53 protein
has been
voted molecule of the year.

5. MECHANISM

It plays an important role in cell cycle control and
apoptosis.
Defective p53 could allow abnormal cells to proliferate, resulting in
cancer.
As many as 50% of all human tumors contain p53 mutants.

In normal cells, the p53 protein level is low. DNA damage and
other stress
signals may trigger the increase of p53 proteins, which have three
major
functions: growth arrest, DNA repair and apoptosis (cell
death). The growth arrest stops the progression of cell cycle,
preventing
replication of damaged DNA. During the growth arrest, p53 may
activate the
transcription of proteins involved in DNA repair. Apoptosis is
the
"last resort" to avoid proliferation of cells containing abnormal DNA.

The cellular concentration of p53 must be tightly regulated.
While it can
suppress tumors, high level of p53 may accelerate the aging process by
excessive
apoptosis. The major regulator of p53 is Mdm2, which can
trigger the
degradation of p53 by the ubiquitin system.

Target Genes

p53 is a transcriptional activator, regulating the expression of Mdm2
(for its own regulation) and the genes involved in growth arrest, DNA
repair and
apoptosis. Some important examples are listed below.

Growth arrest: p21, Gadd45, and 14-3-3s.

DNA repair: p53R2.

Apoptosis: Bax, Apaf-1, PUMA and NoxA.

Regulation of p53

As mentioned above, p53 is mainly regulated by Mdm2. The
regulation
mechanism is illustrated in the following figure.

Figure 1.0.
Regulation of p53.

(a)
Expression of Mdm2 is activated by p53.

(b)
Binding of p53 by Mdm2 can trigger the degradation of p53 via the
ubiquitin
system.

(c)
Phosphorylation of p53 at Ser15, Thr18 or Ser20 will disrupt its
binding with
Mdm2. In normal cells, these three residues are not
phosphorylated, and
p53 is maintained at low level by Mdm2.

(d)
DNA damage may activate protein kinase (such as ATM, DNA-PK, or CHK2)
to
phosphorylate p53 at one of these three residues, thereby increasing
p53 level.
Since Mdm2 expression is activated by p53, the increase of p53 also
increases
Mdm2, but they have no effect while p53 is phosphorylated. After
the DNA
damage is repaired, the ATM kinase is no longer active. p53 will
be
quickly dephosphorylated and destroyed by the accumulated
Mdm2.

Roles of p53

The roles of p53 in growth arrest and apoptosis are illustrated
in Figure
4-H-6. p53 is also directly involved in DNA repair. One of its
transcriptional target gene, p53R2, encodes ribonucleotide reductase,
which is
important for both DNA replication and repair. p53 also interacts
directlywith AP endonuclease and DNA polymerase which are involved in
base
excision repair.

(a)
The cell cycle progression into the S phase requires the enzyme Cdk2,
which can
be inhibited by p21. The progression into the M phase requires
Cdc2 which
can be inhibited by p21, GADD45 or 14-3-3s. p53
regulates the expression of these inhibitory proteins to induce growth
arrest.

(b)
Apoptosis can be induced by the binding of Caspase 9 to cytochrome c
and Apaf1.
p53 may activate the expression of Apaf1 and Bax. The latter can
then
stimulate the release of cytochrome c from mitochondria (see
Mitochondria,
Apoptosis and Aging).

6. ROLE IN DISEASE

If the p53 gene is damaged, tumor suppression is severely reduced.
People who
inherit only one functional copy of p53 will most likely develop tumors
in early
adulthood, a disease known as Li-Fraumeni syndrome. p53 can also be
damaged in
cells by mutagens (chemicals, radiation or viruses), increasing the
likelihood
that the cell will begin uncontrolled division. More than 50 percent of
human
tumors contain a mutation or deletion of the p53 gene.

In health p53 is continually produced and degraded in the cell. The
degradation
of p53 is, as mentioned, associated with MDM-2 binding. In a negative
feedback
loop MDM-2 is itself induced by p53. However mutant p53s often don't
induce
MDM-2, and are thus able to accumulate at very high concentrations.
Worse,
mutant p53 protein itself can inhibit normal p53 (Blagosklonny, 2002).

7. POTENTIAL THERAPEUTIC USE

In-vitro introduction of p53 in to p53-deficient cells has been shown
to cause
rapid death of cancer cells or prevention of further division. It is
more these
acute effects which hopes rest upon therapeutically (McCormick F,
2001). The
rationale for developing therapeutics targeting p53 is that "the most
effective way of destroying a network is to attack its most connected
nodes". P53 is extremely well connected (in network terminology it is a
hub) and knocking it out cripples the normal functioning of the cell.
This can
be seen as 50% of cancers have missense point mutations in the p53
gene, these
mutations impair its anti-cancer gene inducing effects. Restoring its
function
would be a major step in curing many cancers (Vogelstein et al 2000).

Various
strategies have been proposed to restore p53 function in cancer
cells (Blagosklonny,2002).A number of groups have found molecules which
appear to restore
proper
tumour suppressor activity of p53 in vitro. These work by altering the
conformation of mutant conformation of p53 back to an active form. So
far, no
molecules have shown to induce biological responses, but some may be
lead
compounds for more biologically active agents. A promising target for
anti-cancer drugs is the molecular chaperone Hsp90, which interacts
with p53 in
vivo.

Adenoviruses
rely on their host cells to replicate, they do this by
secreting
proteins which compel the host to replicate the viral DNA. Adenoviruses
have
been implicated in cancer-causing diseases, but in a twist it is now
modified
viruses which are being used in cancer therapy. ONYX-015 (dl1520,
CI-1042) is a
modified adenovirus which selectively replicates in p53-deficient
cancer cells
but not normal cells (Bischoff, 1996). It is modified from a virus that
expresses the early region protein, E1B, which binds to and inactivates
p53. P53
suppression is necessary for the virus to replicate. In the modified
version of
the virus E1B has been deleted. It was hoped that the viruses would
select
tumour cells, replicate and spread to other surrounding malignant
tissue thus
increasing distribution and efficacy. The cells which the adenovirus
replicates
in are lysed and so the tumour dies.

Preclinical trials using the ONYX-015 virus on mice were promising
however
clinical trials have been less so. No objective responses have been
seen except
when the virus was used in combination with chemotherapy (McCormick,
2001). This
may be due to the discovery that E1B has been found to have other
functions
vital to the virus. Additionally its specificity has been undermined by
findings
showing that the virus is able replicate in some cells with wild-type
p53. The
failure of the virus to produce clinical benefits may in large part be
due to
extensive fibrotic tissue hindering virus distribution around the
tumour
(McCormick, 2001).